[quote=@Dervish]I myself prefer about 6-8, and as the game's established I don't mind picking up new blood, but my personal GMing style is trying to help players craft their own personal stories and incorporate parts of their character bios into the RP and it's a lot more meaningful and manageable if I only have a small portion of players compared to say an open sandbox with an always open tag.[/quote]

[@Dervish] made a lot of helpful points in his post above; I have some more advices going further into running RPs:

Having an established player base is crucial. However, it's inevitable that some of these players will drop out as the RP progresses. Even if they are fully committed and motivated, their real life may present obstacles we GMs cannot predict. Finding new players is necessary, and I believe doing so serves not only to replace departed players, but also bring in new perspectives to keep the story fresh.

With that said, the longer a thread exists, the harder it is for new players to join in. Besides the obvious need to catch up on story (going anywhere from several pages to the length of several novels), new players also have to contend with a close-knit group that often appears exclusive. Breaking into that established circle is tough; it's awkward and confusing when they laugh at inside jokes and circlejerk over RP-specific memes (even if they are entirely wholesome).

After a certain point (likely after two or three years), it becomes very difficult for the RP to gain new players. That doesn't mean the story has to end. As GMs, we need to be flexible with our plot and group management, and adjust to player-made developments when necessary. This may mean breaking the story down to multiple chapters/arcs/episodes, moving forward with time skips and rebooting to a new thread. 

In the end, remember that a successful RP goes far beyond the original GM(s). I've seen enduring RPs change hands multiple times, letting new voices better the story even as the OP loses interest or resigns due to real life concerns.